All about milk
Milk is a white liquid produced by the mammary glands of mammals. All mammals, including humans, will normally produce milk to feed their offspring until they are ready for solid food.
However, some people are not able to digest lactose, the sugar in milk, after they are weaned, because they do not produce enough of an enzyme known as lactase. Lactase is needed to digest milk properly.
As concerns about lactose intolerance and milk allergies widen, a range of substitute milks, such as almond and soy milk, have become available.
This article, part of a Medical News Today collection of articles on the health benefits of popular foods, will focus mainly on cow's milk.
Health benefits of milk
Milk has long been seen as a healthy drink, because it is high in a range of nutrients. The U.S. Department of Agriculture (USDA) guidelines for 2015 to 2020 suggest that Americans should consume "Fat-free or low-fat dairy, including milk, yogurt, cheese, and/or fortified soy beverages."
Milk and bone health
Cow's milk can be a source of calcium, a mineral that is important in the development and maintenance of healthy bones and teeth.
Milk is good for the bones because it offers a rich source of calcium, a mineral essential for healthy bones and teeth. Cow's milk is fortified with vitamin D, which also benefits bone health. Calcium and vitamin D help prevent osteoporosis.
Other ways to improve bone health and reduce the risk of osteoporosis include regular physical activity and strength training, avoiding smoking and eating a healthy diet that is low in sodium and high in potassium. Most of the body's vitamin D is synthesized by the body on exposure to sunlight, so spending time outdoors is also important.
Some studies have concluded that milk consumption does not improve bone integrity in children.12
In spite of this, milk and milk products are still considered beneficial for bone development in children.
Milk and heart health
Cow's milk is a source of potassium, which can enhance vasodilation and reduce blood pressure.
Increasing potassium intake and decreasing sodium can reduce the risk of cardiovascular disease, according to a study led by Dr. Mark Houston, director of the Hypertension Institute at St. Thomas Hospital in Tennessee.3
The study showed that those who consumed 4069 mg of potassium per day had a 49 percent lower risk of death from ischemic heart disease compared with those who consumed around 1000 mg per day.3
According to the National Health and Nutrition Examination Survey, fewer than 2 percent of US adults meet the daily 4700 mg recommendation.3
Potassium-rich foods include cow's milk, oranges, tomatoes, lima beans, spinach, bananas, prunes, and yogurt. A dramatic increase in potassium intake can have risks however, including heart problems, so any changes in diet or use of supplements must be discussed first with a physician.
Cow's milk also contains a high amount of saturated fat and cholesterol, which have been linked to an increased risk of heart disease.
Milk and cancer
Vitamin D might play a role in cell growth regulation and cancer protection. Research shows that there is a higher risk of dying from colorectal cancer in geographic locations that receive the least amount of sunlight. Milk, too, contains vitamin D that can offer similar protection.
The National Cancer Institute (NCI) states that "Research results overall support a relationship between higher intakes of calcium and reduced risks of colorectal cancer." They note, however, that the results of studies have not always been consistent."2
The NCI also points to some studies that suggest an increased intake of calcium and lactose from dairy products may help to prevent ovarian cancer.2
Milk and depression
Adequate vitamin D levels support the production of serotonin, a hormone associated with mood, appetite, and sleep. Vitamin D deficiency has been associated with depression, chronic fatigue, and PMS. Cow's milk and other foods are often fortified with vitamin D.
Milk and muscle building
Cow's milk is designed to help baby cows grow fast, so it makes sense that humans who drink cow's milk can also bulk up quickly. Cow's milk is a rich source of high-quality protein, containing all of the essential amino acids. Whole milk is also a rich source of energy in the form of saturated fat, which can prevent muscle mass being used for energy.
Maintaining a healthy amount of muscle is important for supporting metabolism and contributing to weight loss and weight maintenance. Sufficient dietary protein is needed to preserve or increase lean muscle mass. Dairy protein can support muscle growth and repair.
According to Today's Dietitian, an analysis of over 20 clinical trials suggests that an increased milk intake can boost muscle mass and strength during resistance exercise in both younger and older adults.6
Cow's milk does not seem to significantly help with weight loss. One analysis of studies found that increased consumption of cow's milk in the short-term and without calorie restriction had no benefit for weight loss, with only modest benefits seen in long-term studies with energy restriction.11
Low-fat milk can provide the benefits of milk while supplying less fat.
Milk and osteoarthritis
Osteoarthritis of the knee currently has no cure, but researchers say drinking milk every day has been linked to reduced progression of the disease. Their research was published in the American College of Rheumatology Journal Arthritis Care & Research.
Nutritional content of milk and milk alternatives
Cow's milk and soy milk contains protein, which supports muscle growth and repair.
One cup of milk is considered one serving. The nutritional breakdown of milk depends on the fat content.
One cup of whole milk, with 3.25 percent fat contains:
- 146 calories
- 8 grams of fat
- 13 grams of carbohydrates
- 8 grams of protein
One cup of nonfat or skim milk contains:
- 86 calories
- 0 grams of fat
- 12 grams of carbohydrates
- 8 grams of protein
In comparison, one cup of plain soy milk contains:
- 80-110 calories
- 3 to 4 grams of fat
- 6 to 7 grams of carbohydrates
- 5 to 7 grams of protein
One cup of plain almond milk contains:
- 50 to 60 calories
- 2.5 grams of fat
- 5 to 7 grams of carbohydrates
- 1 gram of protein
Some important nutrients that all milk provides include:
Calcium: Dairy products like milk are one of the richest dietary sources of calcium. Calcium has many functions in the body but its primary job is the development and maintenance of healthy bones and teeth.
Calcium is also important for blood clotting and wound healing, maintaining normal blood pressure, and muscle contractions including heartbeat. It is important to try to pair calcium-rich foods with sources of magnesium and vitamin D, as vitamin D supports calcium absorption in the small intestine and magnesium helps the body incorporate calcium into the bones.
A cup of skim milk contains around 306 milligrams of calcium, with around 32 percent of this calcium thought to be absorbed. Non-acidifying plant sources of calcium may be preferable for some people, with the absorption of calcium from kale, broccoli and other vegetables ranging from 40 to 64 percent.8,9
Choline: Milk is also a rich source of choline; an important nutrient found to support sleep, muscle movement, learning and memory. Choline helps to maintain the structure of cellular membranes, aids in the transmission of nerve impulses, assists in the absorption of fat and can lessen chronic inflammation.4
Potassium: An optimal intake of potassium is associated with a reduced risk of stroke, heart disease, high blood pressure, protection against loss of muscle mass, preservation of bone mineral density and reduction in the formation of kidney stones. A high potassium intake is associated with a 20% decreased risk of dying from all causes.3 The recommended daily intake of potassium for all adults is 4,700 mg per day.
A cup of cow's milk contains around 366 mg of potassium (slightly more than in most soy milk beverages), although the unpleasant digestive effects of lactose intolerance, such as diarrhea, can lead to potassium depletion.
Vitamin D (fortified): Vitamin D is not naturally present in cow's milk, but it may be added alongside other nutrients to fortify cow's milk, soy milk, almond milk, and other types.
Vitamin D is important for bone health. It aids in the formation, growth, and repair of bones. It also plays an important role in calcium absorption and immune function. Vitamin D deficiency has been associated with osteoporosis, depression, chronic fatigue, muscle pain, PMS, hypertension, and breast and colon cancer.
Milk is also fortified with numerous vitamins, including vitamins A and D. It may also contain small amounts of vitamin B2, or riboflavin, vitamin B12, and around 0.1 milligrams per cup of vitamin B6. Magnesium and phosphorus may also be present. Some of these vitamins, especially A and riboflavin, are destroyed by exposure to light, so milk stored in transparent containers will have lower nutrient levels.
To encourage the consumption of cow's milk, manufacturers have created new products, including flavored varieties like strawberry or chocolate, lactose-free milks, milk with added omega-3s, hormone free or organic milks and reduced fat milk.
However, consumers should remember that some flavored milks can contain high amounts of sugar. It is a good idea to check the labels of foods when looking for healthy options.
Concerns and precautions
Milk alternatives such as soy milk and almond milk may be recommended for people who have lactose intolerance.
Lactose intolerance is a condition in which a person lacks the enzyme lactase, which is needed to break down the sugar found in milk for proper digestion.
Some people, who do not produce enough lactase, cannot tolerate lactose beyond infancy. An estimated 15 percent of people of northern European descent, 80 percent of black and Hispanic people, and more than 90 percent of Asians and First Nations people do not produce lactase.
Lactose intolerance can lead to bloating, flatulence or diarrhea when consuming milk and milk products. The negative effects of lactose intolerance on the gastrointestinal system may compromise absorption of nutrients from other foods.
Drinking lactose-free milk, which has added enzymes to help with lactose digestion, or taking a lactase supplement when consuming milk may ease or eliminate these symptoms.
Milk allergy or hypersensitivity is different from lactose intolerance. It refers to an abnormal immunologic reaction in which the body's immune system produces an allergic antibody, called immunoglobulin E (IgE) antibody.
A cow's milk allergy can cause symptoms such as wheezing and asthma, diarrhea, vomiting, and gastrointestinal distress. Other reactions include eczema, an itchy rash, and rhinitis, or inflammation in the nose. In severe cases, it can lead to bleeding, pneumonia, and even anaphylaxis, a potentially fatal hypersensitivity reaction.
Overconsumption of potassium or phosphorus, both of which exist in high levels in milk, can harm those whose kidneys are not fully functional. If the kidneys cannot remove excess potassium or phosphorus from the blood, it could be fatal.
Overconsumption of calcium is rare with food intake alone, but it can cause unwanted side effects such as constipation, kidney stones, or kidney failure. This may be a risk when taking calcium supplements.
Excess calcium may also increase the risk of calcium deposition in the arteries, raising the risk of heart disease, especially when magnesium intake is low. The tolerable upper intake level of calcium is 2.5 grams per day for healthy individuals over the age of 1 year.
The American Academy of Pediatrics do not recommend cow's milk for infants under 1 year of age. This is partly because cow's milk is low in iron compared with human breast milk. There is also a risk of gastrointestinal bleeding.15,16
Breastmilk is the best choice of milk for infants under 1 year. Introducing cow's milk too early may predispose them to a lactose allergy in future.
This recommendation also stems from evidence that consumption of dairy products in infancy is linked to the development of insulin-dependent (type 1 or childhood-onset) diabetes.
Cow's milk may also contain residues of hormones and antibiotics, as well as dioxins and polychlorinated biphenyls (PCBs). These substances can have a negative impact on human health, including adverse effects on the nervous system, reproductive system, and immune system. They may potentially raise the risk of certain types of cancer.
While calcium and vitamin D from cow's milk can benefit bone health, there is also some evidence that animal proteins in the diet, for example, from cow's milk, have an acidifying effect.
This could have a negative impact on bone health by causing the body to pull calcium from the bones to restore optimal blood pH levels.10 As such, the net benefit of calcium in cow's milk may be much lower than is usually expected.
Plant-based sources of calcium, such as green leafy vegetables, are more effectively absorbed and used than calcium derived from cow's milk.
Anyone who has an allergy or intolerance to cow's milk, or who is considering avoiding cow's milk for ethical or environmental reasons, can find out more about some of the milk alternatives here.